Directional control has become increasingly important in the drilling of subterranean oil and gas wells, for example, to more fully exploit hydrocarbon reservoirs. Two-dimensional and three-dimensional rotary steerable tools are used in many drilling applications to control the direction of drilling. Such steering tools commonly include a plurality of force application members (also referred to herein as blades) that may be independently extended out from and retracted into a substantially non-rotating steering tool body. The blades are disposed to extend outward from the steering tool body into contact with the borehole wall and to thereby displace the steering tool body from the centerline of well bore during drilling. The non-rotating steering tool body is typically deployed about a rotating shaft, which is disposed to transfer weight and torque from the surface (or from a mud motor) through the steering tool to the drill bit assembly.
In order to point (or push) the drill bit in a certain direction, one or more of the blades are moved radially outward into contact with the borehole wall to offset the non-rotating tool body from the centerline of the borehole. In a “point the bit” arrangement, the blades offset the steering tool body in substantially the opposite direction as the direction of subsequent drilling, while in a “push the bit” arrangement, the blades offset the steering tool body in substantially the same direction as the direction of subsequent drilling. Increasing the offset tends to correspondingly increase the degree of curvature (bend) in the borehole as it is being drilled.
While such steering tools are conventional in the art and are known to be serviceable for many directional drilling applications, there is yet room for further improvement. For example, there is a trend in the drilling industry towards drilling smaller diameter boreholes having sections with increased dogleg severity (curvature). As such there is a need for rotary steerable tools capable of achieving higher dogleg Severity (e.g., on the order of 10 or more degrees per 100 feet of borehole).
In conventional rotary steerable tools, as the required dogleg severity (curvature) of a borehole increases (particularly in small diameter boreholes) the trailing end (the upper end) of the non-rotating steering tool body tends to contact the borehole wall and thereby limit the ability of the steering tool to achieve a higher dogleg well path. Moreover, increased dogleg severity increases bending stresses in the steering tool body, Which must be accommodated to prevent tool failure.
Therefore, there exists a need for improved downhole steering tools. In particular, there exists a need for small diameter steering tools capable of achieving high dogleg severity. There also exists a need for a mechanism to accommodate the high bending stresses encountered in high dogleg boreholes.